1. Field of the Invention
The present invention relates to a system for providing a collision warning.
2. Discussion of Prior Art
There is currently world-wide interest in the production of systems for providing a vehicle collision warning. Projected capabilities range from relatively simple "intelligent cruise control" systems, to more complex systems capable of searching the road ahead and alerting/acting on the presence of a potential hazard. For all weather performance, particularly in dense fog, a radar sensor is particularly beneficial.
Considerable problems must be overcome before a truly cost-effective radar based system can be produced. Particularly important problems of implementation are:
a. false alarms from vehicles in adjacent lanes and "road furniture" (street signs, lamp standards etc.); PA0 b. mutual interference between radar systems; PA0 c. reliably discriminating vehicles and structures which are a potential hazard from those which are not an immediate safety threat; and PA0 d. producing a cost-effective hardware configuration which will cope with the above problems PA0 (i) a source of radiation; PA0 (ii) receiving means for receiving radiation emitted by the source and reflected by an object in its field of view and for generating receive signals in response thereto; and PA0 (iii) processing means for processing said signals to determine whether or not an object presents a collision risk, said processing means comprising means for determining from receiver signals a measure of object sightline rate and means for rejecting signals associated with objects having a sightline rate greater than a set value in order to distinguish between objects which do and do not present collision risks, sightline rate being rate of change of angular position with respect to the system, PA0 (a) the receiving means comprises a plurality of receivers; and PA0 (b) the processing means includes combining means for combining signals from different receivers to generate composite signals and provide for the receiving means to have a plurality of polar sensitivity responses associated with respective composite signals such that no direction within the system field of view is associated with a null of every polar sensitivity response. PA0 (a) at least one source of radiation; PA0 (b) receiving means for detecting the radiation after reflection from an object and for generating signals in response thereto; and PA0 (c) signal processing means for determining the sightline rate of an object from the signals;
Several research organizations have been involved in developing collision warning radar systems. A. G. Stove of the Philips Research Laboratories in Electronics and Communication Engineering journal, October 1991, pages 232-240 provides a review of the use of radar in vehicle collision warning systems. Such systems generally comprise a microwave transceiver for transmitting and receiving radar signals, a signal processor for Identifying potential obstacles, and a form of man-machine interface such as a display for informing the driver of the vehicle. The system generates a radar map of all the obstacles within the field of view of the transceiver and then extracts those obstacles which are a potential threat for display.
In certain situations, such as rounding a bend, an obstacle rich environment in front of the vehicle may be measured although there is no real danger presented to the vehicle, provided it negotiates the bend. The signal processing capability required to cope with such an obstacle rich environment is considerable and adds to the complexity and expense of such systems. The vehicle component market is extremely price sensitive and the reduction of unit cost is of critical importance.
European Patent Application number 0 473 866 A2 describes an optical system in which the range and angle of target vehicles are monitored and these are passed to a processor which monitors each target vehicle and determines a predicted separation distance from the target vehicles to the system. Such a system would require a processor having a substantial data processing capacity to cope with monitoring each target vehicle even when the majority of the target vehicles do not present a collision hazard.
It is known, for example in United Kingdom patent specification number 1 313 402 that the necessary and sufficient condition for a collision of a first moving body with a second stationary or moving body is that the relative separation of the bodies decreases with time whilst the relative bearing of the second body measured from the first body remains constant.
United Kingdom patent specification number 1 605 171 describes an aircraft based system for determining the clearance of the aircraft from a landscape feature by determining the angle below the flight line of the aircraft of a sightline from the aircraft to the feature, the ground speed of the aircraft and the rate of increase of the angle of the sightline below the horizontal.
In German patent specification number DE 27 34 998, a Doppler radar system for vehicle collision avoidance is described. The system comprises an emitter/receiver unit comprising a first antenna and a second antenna. The unit is arranged to emit radiation towards a scene in a direction in which a vehicle incorporating the system is traveling. Moreover, the unit is arranged to receive radiation reflected from a target at two spatially separated left and right points on the vehicle, where the first antenna and the second antenna are located respectively, and to generate first and second Doppler signals respectively in response thereto. The first and second Doppler signals have a frequency proportional to relative velocity between the target and the first and second points on the vehicle respectively. The system also incorporates a phase change switch connected to the emitter/receiver unit for determining the phase difference between the first and second Doppler signals, and a switch for producing a reply signal from interrogating the amplitude and phase difference of the Doppler signals. Moreover, the system incorporates a reply signal switch comparator for generating an alarm signal when the reply signal lies within a set range. In operation when the system interrogates the scene, the first antenna emits radiation towards the scene whilst the second antenna receives radiation reflected therefrom to provide the first Doppler signal. The system then commutates so that the second antenna emits radiation towards the scene whilst the first antenna receives radiation reflected therefrom to provide the second Doppler signal. This system has a disadvantage that a commutator is required for switching the antennae which adds to cost of the system.
European Patent Application number EP 0 658 775 A describes an anticollision system for determining variation in sightline angle of potential obstacles to said system. The system incorporates at least two antennae which are arranged to emit radiation sequentially, one after the other. The system also incorporates means for conserving instantaneous phase information relating to echoes received from the obstacles at the antennae to obtain a Doppler signal which contains information relating to sightline angle of the obstacles relative to the system. Moreover, the system employs commutating means for commutating the signals from the antennae; commutation involves switching between the signals received at the antennae, namely keeping the signals separate and selecting amongst them.
A problem with the systems described in German patent specification number DE 27 34 998 and European Patent Application number EP 0 658 775 A is that only a single antenna receives radiation reflected from the scene at any particular time. As a result, a situation may arise where an object on a collision course with one of these systems has a trajectory which is not in a field of view of the antenna in the system arranged to receive radiation. This may result in a situation where the object is not detected and subsequently collides with the system.